What is boron carbide?
Boron Carbide powder belongs to trigonal crystal system. There are 12 B atoms and 3 C atoms in unit cell. The connected stereo diagonal configuration of C atoms in the unit cell is in an active state and can be replaced by B atoms to form a replacement solid solution and may leave the crystal lattice to form high boron compounds with defects.
B4C has a molecular weight of 52.25, C21.74% and B78.25%, usually gray to black, the density 2.519g/cm3, Mohs hardness 9.35, microhardness about 49GPa, second only to the diamond and cubic boron nitride, Therefore, B4C powder has a very high grinding ability, and its grinding efficiency can reach 60%-70% of the diamond, which is higher than 50% of SiC, and is 1-2 times that of corundum.
The melting point of B4C powder is 2450 (decomposition). The coefficient of expansion at 1000degC is 4.5×10-6degC-1. The thermal conductivity is 121.4W/m*k at 100degC and 62.79W/m*k at 700degC. B4C powder is mainly used as abrasives, hot-pressed B4C products can be used as wear-resistant and heat-resistant parts. B4C is mainly used as an additive in the refractory industry, such as being added to carbon-bonded refractories to act as an antioxidant. Unshaped materials improve the strength and corrosion resistance of the body.
Synthesis and typical properties of boron carbide
The commonly used method for synthesizing B4C powder in the industry is to reduce boron anhydride with excess carbon:
2B2O3+7C-B4+6CO|
The synthesis reaction can be carried out in a muffle furnace or an electric arc furnace. When synthesizing in a resistance furnace, heating the mixture of boron anhydride B2O3 and carbon C at a decomposition temperature lower than B4C can obtain B4C (sometimes containing Free B 1%-2%) is a better synthesis method. When it is synthesized in the electric arc furnace, because the arc temperature is quite high, B4C powder is decomposed into carbon-rich phase and boron at about 2200. Sometimes it will volatilize and cause a large amount of free C (20%-30%) in the reaction product, so the quality of the obtained B4C is slightly worse.
When synthesizing B4C by electric arc furnace, boric acid (content greater than 92%), artificial graphite (fixed carbon greater than 95%) and petroleum coke (fixed carbon greater than 85%) are usually used as raw materials, and the theoretical dosage is calculated according to the reaction formula. The theoretical amount is about 2% higher. Artificial graphite powder and petroleum coke each account for 50% of the total carbon added amount, and then 3%-4% higher than the theoretical amount. Mix the prepared three raw materials in a ball mill and add them to the arc B4C can be obtained by reducing and carbonizing the furnace at 1700-2300. Finally, the frit is sorted and washed, crushed, ground, pickled, sedimentation and classification, etc., to obtain B4C of various particle sizes.
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